The invention relates generally to the time synchronization of measurements taken by multiple measurement devices, and more particularly to the time synchronization of measurements taken by multiple metrology devices such as articulated arm coordinate measuring machines, laser trackers, laser scanners, and other types of parts measurement precision metrology devices.
For example, one of these metrology devices may belong to a class of instruments that measure the coordinates of each of a number of points on an object or part to be measured by probing the point with an articulated mechanical structure. The probing may be performed with a contacting mechanical probe tip and/or with a non-contacting scanning device (e.g., a laser line probe (LLP)). The position of the probe tip or scanning device relative to the base of the structure (i.e., in a certain coordinate frame of reference) is determined by the readings of angular encoders located at the connection points (e.g., bearing cartridges) of the articulating arm segments. This type of device, whether it uses a mechanical probe tip or a scanner, is referred to as an articulated arm coordinate measuring machine.
Portable articulated arm coordinate measuring machines (AACMMs) have found widespread use in the manufacturing or production of parts where there is a need to rapidly and accurately verify the dimensions of the part during various stages of the manufacturing or production (e.g., machining) of the part. Portable AACMMs represent a vast improvement over known stationary or fixed, cost-intensive and relatively difficult to use measurement installations, particularly in the amount of time it takes to perform dimensional measurements of relatively complex parts. Typically, a user of a portable AACMM simply guides a probe along the surface of the part or object to be measured. The measurement data are then recorded and provided to the user. In some cases, the data are provided to the user in visual form, for example, three-dimensional (3-D) form on a computer screen. In other cases, the data are provided to the user in numeric form, for example when measuring the diameter of a hole, the text “Diameter=1.0034” is displayed on a computer screen.
An example of a prior art portable articulated arm CMM is disclosed in commonly assigned U.S. Pat. No. 5,402,582 ('582), which is incorporated herein by reference in its entirety. The '582 patent discloses a 3-D measuring system comprised of a manually-operated articulated arm CMM having a support base on one end and a measurement probe at the other end. Commonly assigned U.S. Pat. No. 5,611,147 ('147), which is incorporated herein by reference in its entirety, discloses a similar articulated arm CMM. In the '147 patent, the articulated arm CMM includes a number of features including an additional rotational axis at the probe end, thereby providing for an arm with either a two-two-two or a two-two-three axis configuration (the latter case being a seven axis arm).
Another type of these devices may be an instrument, referred to as a laser tracker, which measures the coordinates of a point by sending a laser beam to a retroreflector target that is in contact with the point. The laser tracker determines the coordinates of the point by measuring the distance and the two angles to the retroreflector. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Exemplary systems belonging to this class of instruments are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al.
A laser tracker can measure relatively large parts (i.e., parts larger than what a portable AACMM can measure without relocation of the AACMM) and in a shorter amount of time as compared to a portable AACMM, especially if the portable AACMM must be relocated to completely measure the part. Modern laser trackers can measure relatively large parts with 3-D single-point accuracy to 0.001 inches. Such a laser tracker typically uses its laser to measure the 3-D coordinates or a point at a range of up to 230 ft (70 m) by following the movement of a retroreflector such as a spherically mounted retroreflector (SMR) and report on the measured position in real time. Some modern laser trackers can provide real time updates of SMR positions.
There are several types of laser scanners, but all types project light onto objects to be tested or measured. Most surfaces of these objects are diffusely scattering, and measurements are ordinarily made without the assistance of a cooperative target such as a retroreflector. Some scanners, such as the one that attaches to the end of the AACMM described above (e.g., an LLP), are moved by hand and the laser light is directed over a surface of the object. The contacting probe of the portable AACMM and the non-contacting laser line probe can digitize data interchangeably without having to remove either component from the AACMM. A laser line probe provides multiple sampling points to be taken simultaneously along an object's surface, which is illuminated by the laser scan line. Users can accurately measure prismatic features with the AACMM contacting probe, then laser scan sections requiring larger amounts of data (detailed feature extraction) at more than 19,000 points per-second—without adding or removing attachments. An exemplary non-contacting scanner of this type is described in commonly assigned U.S. Pat. No. 6,965,843 to Raab et al., which is incorporated herein by reference in its entirety.
Other scanners are stationary and illuminate an entire area of interest. An exemplary scanner of this type is described in U.S. Pat. No. 7,599,071 to Dillon et al. A third type of laser scanner sends a laser beam over a scan pattern that covers a large volume. A laser scanner of this type may sometimes measure the 3-D coordinates of a relatively large volume within a few minutes. An exemplary laser scanner of this type is described in commonly assigned U.S. Pat. No. 7,430,068 to Becker et al., which is incorporated by reference herein in its entirety.
There are many situations in which multiple metrology devices, either all of the same type or of different types, being connected, arranged or combined in a distributed networked system, may benefit from being synchronized in time to one another. In general, as distributed network technologies increase in number and complexity, these system technologies applied to measurement and control become more complex as the number of nodes (i.e., metrology devices) in the system increases. It has become common to utilize local real time clocks in the various devices to achieve relatively accurate system-wide time. However, each of these individual clocks tends to drift apart from one another (i.e., lose their system-wide synchronization) due to, for example, initial frequency offsets, instabilities in the clock oscillators and environmental conditions such as temperature, aging, vibration, mechanical stresses, etc. As such, the measurements taken by these various devices and any resulting control imparted to the overall system suffer from the time inaccuracies of the individual clocks. Thus, some type of clock time synchronization correction or adjustment is needed to properly synchronize the individual clocks to thereby maintain an accurate and common measure of system-wide time.
The following outlines five examples in which precise time synchronization benefits system combinations of precision metrology devices.
Tracker and Arm: The portable AACMM is capable of being moved or positioned into a variety of different orientations. Because of this, the portable AACMM is able to measure “hidden” points; that is, points that are hidden from a line-of-sight view of a measuring device such as a laser tracker. On the other hand, the laser tracker can measure over a much larger volume than the AACMM. It is possible to use a laser tracker to relocate a portable AACMM by attaching a retroreflector to the arm portion of the AACMM. In this way the best features of each device are retained, while enabling the combined system to accurately measure hidden points over a relatively large volume.
An exemplary method for physically relocating the portable AACMM in this way is described in commonly assigned U.S. Pat. No. 7,804,602 to Raab, which is incorporated by reference herein in its entirety. To obtain the best possible relocation, it is important that the portable AACMM and the laser tracker be accurately synchronized in time to one another so that the corresponding measurements made by each device (e.g., of the position of the retroreflector) are accurately synchronized during the relocation process.
Simultaneous multilateration: By using the highly accurate distance meters (e.g., interferometers or absolute distance meters) of a multiple of laser trackers to simultaneously measure a single, wide-angle retroreflector target, the 3-D coordinates of the target can be measured to a relatively high degree of accuracy. It is desirable to make such a simultaneous multilateration measurement on a moving target. This enables the target to be moved over surfaces of objects of interest, thereby producing a map of the surface contours. To retain the relatively high accuracy obtained with this method, the multiple trackers must be accurately synchronized in time.
Wireless synchronization of scanner on end of AACMM: Advances in technology have caused a rapid increase in the rate of data collected by scanners attached to AACMMs, typically at the probe end. This increasing data rate is making it relatively more difficult to successfully transmit data from the scanner located at one end (e.g., the probe end) of the AACMM to the computing elements typically located within the base at the other end of the AACMM. A way around this problem is to send data wirelessly from the scanner to the computer or computing elements. For this wireless approach to be practical, it is important that the part measurement data from the scanner be accurately synchronized with probe positional data from the angular encoders within the AACMM.
Compensation and calibration of laser scanner: It is often necessary to compensate or calibrate a laser scanner of the type described above which sends a laser beam over a scan pattern that covers a relatively large volume. This type of scanner may sometimes measure 3-D coordinates of such a large volume within a few minutes. One way to do this is to compare readings of the scanner to those of a more accurate instrument such as a laser tracker. This may be done by placing a target on a carriage mounted on a motorized rail. The scanner is placed on one end of the rail and points its laser beam to a suitable diffuse target. A laser tracker is placed on the other end of the rail and points its laser beam to a suitable retroreflector target. The diffuse and retroreflector targets are placed back to back and face in opposite directions. To speed up the collection of data, it is desirable to collect data from both the scanner and the tracker while the target assembly is moving. This is only possible if the scanner and tracker are accurately synchronized.
Compensation of AACMM: It is possible to find compensation parameters of an AACMM by moving the arm segments of the articulated arm portion of the AACMM into a variety of different positions while a laser tracker follows a retroreflector target attached to the arm. Compensation parameters are found by comparing the 3-D readings from the laser tracker to readings of encoders in the AACMM. For this method to be practical, the arm segments must be moved into a wide variety of positions while the tracker records retroreflector position data at a relatively high rate. This method is accurate only if the laser tracker and the AACMM are accurately synchronized.
In all of the above example cases, what is needed is a way to synchronize the measurements taken between multiple metrology devices—in some cases, different types of multiple metrology devices, in other cases, similar types of multiple metrology devices.
Time synchronization is necessary and is already available within a portable AACMM. For example, the plurality of angular encoders used in an AACMM are sampled simultaneously. Similarly, the laser interferometer and absolute distance meter in a laser tracker are sampled in such a way that measurement results for each correspond to the same instants in time. Such synchronization is relatively easy to do in a single piece of equipment as a common clock is available. However, this situation changes when multiple portable AACMMs and/or other measurement devices are used in a coordinated measurement manner. In this case, typically a master clock is provided and a means by which the devices are synchronized to the master clock is also provided.
Previous methods of time synchronization of precision measurement devices have an inherent problem of a relatively large and unacceptable synchronization error. To demonstrate the problems created by synchronization error, consider the case where the time error is one millisecond in synchronization. If one of the measurement devices in a system of multiple measurement devices is a laser tracker measuring a retroreflector moving at one meter per second, the resultant error in measured distance is one millimeter. However, if an accuracy of ten micrometers is needed, the error is one hundred times larger than acceptable. Also, for that situation (as in a prior art case) without any synchronization of the clocks driving the individual metrology devices, the fact that a measurement has to wait for the respective “sync” signals to occur introduces additional synchronization delays. For example, an additional millisecond may be added to the total propagation delay.
For example, one prior art system used one laser tracker as a master tracker and a second laser tracker as a slave tracker. The master sent out a strobe signal that was received by the slave. The strobe was sent through cable drivers and was intercepted by receivers in the tracker. This signal went to a microprocessor that implemented a measurement task on that slave tracker as well as the master tracker. This process caused about one millisecond of delay. Cable propagation and capacitance/resistance of the circuit components accounted for much of this delay.
The IEEE 1588 precision time protocol (PTP), which implements a master clock in a phase locked loop manner, has also been proposed for use in robotics, for example as discussed in U.S. Published Patent Application No. 2006/0287769 to Yanagita. However, Yanagita at paragraph [0007] does not in fact recommend use of IEEE 1588. Instead, Yanagita states that IEEE 1588 requires specialized and expensive hardware, and instead proposes a programmed software solution using a master and slave tick count for synchronization of two or more robotic arms. In general, robotic devices used in manufacturing seldom have a need to achieve the 500 ns (nanosecond) time synchronization accuracy required by metrology instruments for applications of the types described above. Thus, the fields of robotics and metrology do not have as much in common as a cursory examination might suggest.
What is needed is apparatus and a method for relatively precise time synchronization (e.g., 500 nanoseconds maximum) of the real time clocks within a multiple of similar or different metrology devices using the IEEE 1588 precision time protocol (PTP), to thereby precisely synchronize the measurements taken by the metrology devices to a desired, relatively high level of accuracy.